1. Field of the Invention
The present invention relates to an electronic expansion valve, which is particularly used in the refrigeration cycle with a compressor and more particularly to an electronic expansion valve in which a stepping motor is used to regulate expansion valve opening in response to the output signal from a digital control unit in such a way that the stable operation of the refrigeration cycle is achieved. Furthermore, the orifice contour created by a moving valve body and a valve seat is always maintained streamlined so that the friction loss therethrough is substantially reduced, whereby the coefficient of performance (COP) of the refrigeration cycle is improved.
2. Description of the Prior Art
In recent years, heat pumps are gaining popularity which provides both heating and cooling modes. They basically consist of two heat exchangers, a compressor and an expansion valve. However, both the nominal evaporating temperature and the nominal pressure difference across the orifice of the expansion valve are different between the heating and cooling modes. Even in the case of the heating mode, a necessary valve opening varies depending upon an outdoor temperature and a desired indoor temperature.
Automatic expansion valves are generally used in the conventional heat pumps which may be classified into a mechanical type and an electrical type. The mechanical expansion valves are further categorized into a constant pressure type and a constant temperature type and are widely employed in medium to large size heat pumps. The valve opening is controlled by balancing the force created by a diaphragm, bellows or a spring and the force resulting from the pressure in an evaporator or the pressure difference between the evaporator and the the compressor inlet.
These mechanical expansion valves are typically designed and constructed for a given rotational speed or capacity of a compressor employed. Therefore, the efficiency of the heat pumps is optimum only for a predetermined external and load condition. When actual operating conditions should change, the efficiency of the heat pumps would decrease in general. Because of the difference in the operating conditions, it is not preferable to use the same mechanical expansion valve in both the heating and cooling modes. Instead, two expansion valves are generally used in this situation.
Recently, solar heat pump systems are becoming popular. They utilize a solar collector as the evaporator of the heat pump. In the heating mode, the solar energy is absorbed by the solar evaporator. In the cooling mode, the heat can be dissipated into the sky during nighttime using the same solar evaporator. The overall efficiency of the heat pump system with the solar evaporator can be substantially improved under these conditions. However, the expansion valve must be operated under widely changing external conditions. The mechanical expansion valve cannot meet this requirement.
Moreover, since the mechanical expansion valve has a non-linearity such as hysteresis, it does not offer a high degree of control accuracy. In addition, as there is no damping means, the refrigeration cycle tends to become unstable.
In order to resolve the above mentioned problems peculiar to the mechanical expansion valve, electric expansion valves are developed, in which the position of the valve body is controlled electrically in the following manner. State variables of the control system which are obtained by sensors such as temperature sensors are once converted into electrical analog signals. They in turn are passed through an electrical network, amplified and applied to a heater or a solenoid, controlling the position of the valve body. As compared with the mechanical expansion valve, the electric expansion valve has a wider dynamic range and can operate under various external conditions. Therefore, by using the electric expansion valve, the heat pump systems can be designed to cover wider operating conditions than is possible using the conventional mechanical expansion valve.
However, since the control law for the conventional electric expansion valve is incorporated in a hardware as an electric circuit, the actuator can perform only a simple operation. In the case of the thermoelectric expansion valve, since the actuator has a large time constant due to combined action of a heater and a bimetal, its response characteristic is unsatisfactory. Moreover, as the valve actuation is performed by a rectilinear motion, a high driving power is required. As a result, a direct current power supply for the heater must be large in capacity. Therefore, both the electric circuit and the actuator become bulky and complicated in structure.
In order to overcome some of the above problems, motor driven expansion valve is disclosed in U.S. Pat. No. 2,534,455 entitled "Refrigerating Control Apparatus", granted to L. B. Koontz to reduce the driving force. According to Koontz Patent, two sets of heat-responsive resistor and heater are provided at two different places along an evaporator coil in which the refrigerant passes and they constitute a bridge circuit for the measurement and actuation. In response to an unbalanced current produced when the superheat temperature deviates from a predetermined value, the motor is driven. The rotational motion of the motor is converted into the translational motion for opening or closing the expansion valve.
However, there are still some problems with the motor driven electric expansion valve. They are tabulated below:
(1) In general, these electric expansion valves cannot be closed completely. Therefore, a solenoid valve is required to close the refrigeration cycle. Furthermore, when the expansion valve is fully opened, the pressure difference across the valve cannot be made substantially zero. PA0 (2) These electric expansion valves are generally of unidirectional type. Two expansion valves must be used for the heating and cooling modes. Alternatively, a four-way valve must be employed to make the flow in the valve unidirectional. PA0 (3) The orifice of the valve has typically a sharp edge. Therefore, the friction loss therethrough is high and consequently the COP of the heat pump reduces. Since the flow coefficient of the orifice is low and the pressure difference created by a linear valve motion is large, the control of the refrigeration cycle tends to be unstable. PA0 (4) Since the relationship between the pressure difference and the valve stroke is non-linear, a better control law is hardly implemented. PA0 (5) An axial force is created across the expansion valve because of the pressure difference. In order to obtain a sufficient torque to overcome this force, a reduction gear may be incorporated between the motor and the valve body. As a result, the mechanism becomes complicated and problem arises with the controllability and response. PA0 (6) It is hard to modify or change the control law which is programmed by hardware in the control unit. There is a limitation of further improving the controllability and efficiency.